Standard electromagnetic stirring operations of the type to which the instant invention pertains comprise exposing the product being cast to one or more mobile magnetic fields that move in a predetermined direction and that act on the liquid metal to move same in the same direction as the field.
In the case of continuously cast strands of elongate sections destined to form slabs, the molten metal is made to move horizontally parallel to the wide faces of the strand.
The mobile magnetic field is normally created by a multiphase static inductor positioned immediately adjacent the cast product, of any of several designs such as, for example, a monobloc inductor of the type used in the stator of a linear-induction motor and placed behind the rollers that hold and guide the strand during casting, or instead used as one or more of these rollers (see French Pat. No. 2,068,803 and German Pat. No. 2,401,145), or placed in the gap between adjacent rollers (see French Pat. No. 2,187,468). It has also been proposed to use a cylindrical inductor which is fitted inside a tubular strand-guiding roller (see British Pat. No. 1,405,312).
The advantage of controlled stirring of the molten metal during casting, which has been recognized for a long time, is in the improved internal quality of the stirred product as compared to an unstirred product. This improved quality, which is characterized in particular by a reduction in central porosity as well as by a substantial reduction of axial macrosegregations, is created by the favorable influence of the stirring on the structure as it solidifies. This latter in fact shows how in stirred products there is a premature interruption of the peripheral crystalline "basaltic"-type or dendritic growth in favor of more formation and development of a central zone with an unoriented solidification structure, that is of the so-called "equiaxial" type.
Nonetheless, although the interrelation between cause and effect between a wide equiaxial zone and a small axial segregation cannot be denied, numerous metallographic observations of the instant inventors show that the axial segregation can nevertheless remain relatively great even with a well developed equiaxial zone.